A DC-DC converter converts a direct current input voltage to a different direct current output voltage and outputs the direct current output voltage to a load. A DC-DC converter is included in each electronic circuit operating with different direct current voltages in various electronic products such as laptop computers, and converts an input voltage to a stable direct current voltage required by the electronic circuit and outputs the direct current voltage. DC-DC converters are classified into insulated DC-DC converters and non-insulated DC-DC converters based on operating principle. An insulated DC-DC converter increases and decreases an input voltage by a transformer. A non-insulated DC-DC converter interrupts a current flowing in an inductor by a switching transistor, and converts a direct current input voltage to a direct current output voltage with different voltage and polarity. In electronic circuits with similar input voltage and output voltage, non-insulated DC-DC converters are used which can be made of relatively simple circuit elements.
Non-insulated DC-DC converters are further classified into step-up DC-DC converters that increase a direct current input voltage to generate a direct current output voltage, step-down DC-DC converters that decrease a direct current input voltage to generate a direct current output voltage, and inverted DC-DC converters that invert the polarity of a direct current input voltage to generate a direct current output voltage.
A related step-down DC-DC converter 100 will be described below with reference to FIG. 7. A direct current input power supply 30 generates a direct input voltage Vi between a high-tension side supply terminal 30a and a low-tension side supply terminal 30b. A diode D1 of which forward direction is from a low-tension side to a high-tension side and a switching transistor Tr1 are serially connected between the high-tension side supply terminal 30a and the low-tension side supply terminal 30b to form a closed circuit as illustrated in the figure.
A connecting point A1 between the diode D1 and the switching transistor Tr1 is connected to a high-tension side output line 32 with the other side being a high-tension side output terminal 32a via an inductor L1, and a connecting point between the diode D1 and the low-tension side supply terminal 30b is connected to a low-tension side output line 33 with the other side being a low-tension side output terminal 33a. A capacitor C1 for outputting a stable output voltage Vo to a load RL connected between the high-tension side output terminal 32a and the low-tension side output terminal 33a is connected between the high-tension side output line 32 and the low-tension side output line 33.
The switching transistor Tr1 is an FET (field effect transistor) for example, and its switching is controlled by a driving signal output to a gate of the switching transistor Tr1 from a constant voltage control circuit 40. During the switching transistor Tr1 is controlled to be closed (controlled to be on) and is operating in a saturation state, a current flows from the direct current input power supply 30 to the inductor L1 to charge the capacitor C1, but a charging voltage of the capacitor C1 that is the output voltage Vo becomes a voltage lower than the input voltage Vi by self-induction of the inductor L1. During the switching transistor Tr1 is controlled to be open (controlled to be off) and is operating in a cut-off state, electrical energy stored in the inductor L1 becomes a charging current that circulates via the diode D1 to charge the capacitor C1 and maintains a charging voltage of the capacitor C1 that is the output voltage Vo.
Since the output voltage Vo can be controlled by closing control time of the switching transistor Tr1 in a unit time, the constant voltage control circuit 40 negatively feeds back an on-duty of a driving signal that closes the switching transistor Tr1 from the output voltage Vo, and performs constant-voltage control of the output voltage Vo so that the output voltage Vo becomes an operating voltage of the load RL. Therefore, the constant voltage control circuit 40 includes a pair of divider resistors R1 and R2 connected between the high-tension side output line 32 and the low-tension side output line 33, and a voltage of a connecting point of the divider resistors R1 and R2 and a reference power supply voltage Vref adjusted to a predetermined electric potential based on an operating voltage of the load RL are compared by a comparator 41 and supplied to a pulse width modulator PWM. The pulse width modulator PWM modulates a pulse width of a transmission signal with a certain period output from a transmitter OSC by a comparison signal of the comparator 41 to output the transmission signal to a driving circuit 42, and the driving circuit 42 outputs a driving signal of which on-duty is adjusted in accordance with the comparison signal of the comparator 41 to a gate of the switching transistor Tr1. Accordingly, when the output voltage Vo is higher than an operating voltage of the load RL for example, since a driving signal with lowered on-duty is output from the driving circuit 42 to the gate of the switching transistor Tr1 and on-control time in the unit time is shortened, the output voltage Vo is lowered. In contrast, when the output voltage Vo is lower than the operating voltage of the load RL, since a driving signal with increased on-duty is output to the gate of the switching transistor Tr1 and on-control time in the unit time is extended, the output voltage Vo is increased. Therefore, the output voltage Vo is controlled to be a constant certain operating voltage different for each of the loads RL.
Generally, a DC-DC converter of this kind is provided with a protection circuit for detecting decrease in an output voltage and abnormal increase in an output current to interrupt the output lines 32 and 33 since a circuit of the load RL may be broken or fire may occur in an unexpected abnormal operation state such as overload and short circuit of an output line (PATENT LITERATURES 1 to 3).